1. Field of the Invention
The present invention relates to a thin type speaker, and more specifically to a speaker having a diaphragm of which the surface is covered with a secondary diaphragm. The secondary diaphragm can enhance the power capacity of the speaker and gives the speaker a larger flat effective bandwidth, with less noise. The speaker can be produced at low cost.
2. Description of the Prior Art
A conventional thin type speaker, as shown in FIGS. 1 and 2, is used in earphones, notebook computers, or miniature type radios and cassette recorders. The prior art speaker is generally composed of an enclosure 10, a mylar cone diaphragm 20, a yoke 30, a sound coil 40, a magnet 50, a washer 60 and a tab 70. The yoke 30 and the enclosure 10 can be integrated into a single piece. The diaphragm 20 is made of plastic material. Such a structure and materials were not changed in the past. After a wide application in the field, it becomes evident that the power capacity of the thin type speaker is rather small (around 0.3 watt). The vibration of the diaphragm 20 often produced noises and the fo value is high while the low frequency effect is poor. Also, the effective bandwidth is small (approximately between 1200 Hz and 10000 Hz). The dB value is around 83.4 dB at 1.26 volts (refer to FIG. 3 for details). Thus the actual sounding effect is not satisfied.